Hitherto, when D.C. (direct current) motors are employed for zooming or focusing lenses or driving an iris in CCTV cameras, if, for example, in a zooming operation wherein a mechanical stopper is installed so that a zoom lens group may move to a telephoto extremity or a wide-angle extremity, a mechanical torque limiter and a friction clutch are installed in order to protect the direct current motor; and a current limit circuit by means of a micro-switch or the like is installed for the purpose of protecting the driving circuit of the direct current motor.
In FIGS. 5, 6, and 7, a simplified wiring diagrams for the driving circuit for a conventional CCTV motorized zoom lens of the prior art are illustrated.
The embodiment of the prior art constitutes a zoom mechanism which is driven via a lens motor; wherein the lens motor is a D.C. motor. The power terminals A and B are connected to a pair of input terminals of the D.C. motor via the wide-angle limit-switch SWw and the telephoto limit-switch SWt, respectively. The wide-angle limit-switch SWw is a current-breaker micro-switch which detects when the zoom lens group has reached the wide-angle extremity. Similarly, the telephoto limit-switch SWt is a current-breaker micro-switch which detects when the zoom lens group has reached the telephoto extremity. In each limit-switch, there is connected in parallel a `reverse` current protector diode (D1 and D2), namely, limit-switches SWw and SWt by-pass the normal close terminal NC and common terminal C via diodes D1 and D2 from power terminals A and B in a `forward` direction.
When the zoom lens group has not arrived at either the telephoto end or the wide-angle end, that is, when the zoom lens is at an intermediate focus position between the telephoto end and the wide-angle end, both of the limit switches SWw and SWt are ON (see FIG. 5). Namely, both of the power terminals A and B are connected to an input terminal of lens motor M via limit switches SWw and SWt. Accordingly, as current can flow in any direction from power terminals A and B in response to the polarity of the voltage applied to power terminals A and B, it is also possible for the lens motor M to rotate in any direction.
The relationship between the voltage at the power terminals and the rotational direction of the lens motor M at an intermediate position, in the above mentioned circumstances, is shown in the following table.
TABLE 1 ______________________________________ Voltage at Power Voltage at Power Rotational Direction Terminal A Terminal B of Lens Motor ______________________________________ + 0 Telephoto Direction 0 + Wide-angle Direction ______________________________________
The lens motor M in the conventional example, as shown in FIGS. 5, 6 and 7, rotates in the telephoto direction when the power terminal A is positive, that is, when the current flows from the terminal A to the terminal B; and rotates in the wide-angle direction when the power terminal B is positive, that is, when the current flows from terminal B to terminal A.
If the lens motor M is rotated in the telephoto direction by means of flowing current from terminal A to terminal B, the limit switch SWt at the telephoto extremity is OFF when the zoom lens reaches the telephoto extremity (see FIG. 6).
The relationship between the voltage at the power terminals and the rotational direction of the lens motor M at a telephoto extremity is shown in the following table.
TABLE 2 ______________________________________ Voltage at Power Voltage at Power Rotational Direction Terminal A Terminal B of Lens Motor ______________________________________ + 0 Stopped 0 + Wide-angle Direction ______________________________________
When the limit switch SWt at the telephoto extremity is OFF, the current does not flow from terminal A in the direction of the lens motor M. Namely, the driving current toward the telephoto direction is cut and, as a result, the lens motor M is stopped. However, as the current flows from the power terminal B through the lens motor M in the direction of the power terminal A via diode D2, zooming in the direction of the wide-angle becomes possible.
When the lens motor M is rotated in the wide-angle direction by flowing current in the direction from terminal B to terminal A, the limit switch SWw at the wide-angle is OFF when the zoom lens arrives at the wide-angle extremity (see FIG. 7). The relationship between the voltage at the power terminals and the rotational direction of the lens motor M at a wide-angle extremity is shown in the following table.
TABLE 3 ______________________________________ Voltage at Power Voltage at Power Rotational Direction Terminal A Terminal B of Lens Motor ______________________________________ + 0 Telephoto Direction 0 + Stopped ______________________________________
When the limit switch SWw at the wide-angle side is OFF, current cannot flow from terminal B to terminal A. Namely, the driving current at the wide-angle direction is cut, and as a result, the lens motor M stops. However, as current flows from terminal A through the lens motor M in the direction of terminal B via the diode D1, zooming in the direction of the telephoto is still possible.
However, in a movable body such as a zoom lens group, the faster the travel speed, the more inertia is incurred. Upon collision of the moving body with a mechanical end-stopper, an extremely large amount of force is exerted on the drive components and on the drive shaft of the D.C. motor. Not only are there problems of durability, the problem of the moving body rebounding in the opposite direction also becomes an issue.
Moreover, if a mechanical switch such as a micro-switch is employed as a limit-switch for a zoom lens, there is a problem with the rebound force further increasing upon contact due to the influence of the spring and actuator. In the prior art, various components have to be strengthened in order to allow a high travel speed of a moving body such as a zoom lens.
In view of the problems mentioned concerning the driving of a conventional CCTV motorized zoom lens, it is an object of the present invention to provide a limit circuit for a D.C. motor having a reduction in rebounding, caused by the inertia of the moving body, and an increased durability thereof.